Gennett ranged to his left for a ball off the bat of Yasmani Grandal as Grandal led off the second inning Friday. Gennett slid to make the stop and threw to pitcher Robert Stephenson covering first for the out. Gennett went to his knees and was helped off by Reds head athletic trainer Steve Baumann and manager David Bell.
"I don't know exactly if it was when he fell or when he made the throw,'' Bell said. "He felt something in his groin."
Cincinnati said Gennett was to have an MRI.
The 28-year-old Gennett reached the majors with the Brewers in 2013 and was claimed off waivers by his hometown Reds near the end of spring training two years ago. He made his first All-Star team last season, in which he hit .310 with 23 homers and 92 RBIs.
Top prospect Nick Senzel could be a candidate to replace Gennett. Senzel was reassigned to minor league camp earlier Friday.
The Associated Press contributed to this report.